The present invention relates to a servoamplifier in an automatic diaphragm controlling device of a camera.
In general, two systems have been used in operating the automatic diaphragm controlling device of a camera, i.e., a single driving coil system of a servomotor for driving the diaphragm wherein the direction of flow of the current in the coil is switched from one direction to the opposite direction with the amount of the current varying accordingly depending upon the result of the measurement of the scene brightness so as to actuate the diaphragm coupled with the servomotor in one direction or in the other direction for achieving the proper exposure, and two coil system wherein one of the coils is energized to open the diaphragm by the current flowing therethrough while the other coil is energized to close the diaphragm by the current flowing therethrough, the difference in the amounts of the currents for the energization of both coils being dependent upon the result of measurement of the scene brightness so that the diaphragm is controlled for the proper exposure depending upon the scene brightness when the diaphragm is moved to a position wherein both currents are equalized.
In the two coil-system, provision of a single electric source suffices to operate the same and balance in the device is obtained when the current flowing in each coil is made equal to each other, and it is relatively easy to construct such a device. However, it has disadvantage that consumption of electric power is great, because current is flowing at all times in each coil.
It has also been proposed to reduce the current flowing in each coil to zero, when balance is achieved, while the nonsensitive range is simultaneously reduced. However, such a measure requires a very complicated construction of the device thereby rendering the device to be difficult to be manufactured.
On the other hand, a single coil system requires a pair of electric sources, but the current flown through the coil is reduced to zero when balance is achieved thereby permitting the power to be saved, and such a device is relatively easy to construct. However, middle point of the electric source is required so that even number of the batteries must be provided, and a pair of power switches are required the sequence of operations of which is determined thereby rendering the manipulation to be inconvenient. In case the electric source is used commonly for the automatic diaphragm controlling circuit and for the film feeding circuit as in the case of 8mm cinecamera, variation in the voltage occurs at one terminal of the electric source due to noise from the film feeding circuit because one terminal of the servomotor is directly connected to the electric source, so that the operation of the diaphragm controlling circuit is affected by the noise.
To avoid the above disadvantages, one coil--one electric source system has also been proposed. However, such a system requires a complicated construction and renders the cost for manufacturing the same is rendered to be high.